The present invention relates to a chip variable resistor.
A chip variable resistor includes, as essential structural parts, an insulating substrate having an upper surface formed with a resistor film in the form of a strip, while also including a rotor having a contact portion to come into contact with the resistor film. The resistance is adjustable by moving the contact portion of the rotor in the longitudinal direction of the resistor film.
Conventionally, as disclosed in e.g. JP-A-11-297517 as prior art, the insulating substrate is formed with a center hole vertically penetrating the substrate, and the rotor is made of a metal plate into a bowl-like shape opening upward. The terminal plate arranged on the lower surface of the insulating substrate is provided with a center cylinder fitted into the center hole and extending upward while penetrating through the rotor. By spreading the upper end of the center cylinder by crimping, the rotor is held in a rotatable manner, and the terminal plate is held not to drop from the insulating substrate.
Further, the terminal plate includes an electrode (center electrode) formed by bending the plate and exposed to the outside of the insulating substrate. The resistor film is horseshoe-shaped in plan view, including an arcuate portion surrounding the center hole of the insulating substrate. The insulating substrate is provided with a first electrode electrically connected to one end of the resistor film and a second electrode electrically connected to the other end of the resistor film.
The rotor overlaps the insulating substrate at a portion inward of the arcuate portion of the resistor film and includes a contact portion projecting downward for contact with the arcuate portion of the resistor film. The rotor is formed with an engagement hole in the form of a cross or a bar into which a driver for operating the rotor for rotation is to be fitted.
The dimensions of the chip variable resistor are set so that the length of one side is no more than 2 mm, for example. Since both of the rotor and the terminal plate need be processed into complicated configurations according to the structure of the prior art, the processing tends to take much time and can be troublesome.
Further, in accordance with the size reduction of electronic devices in recent years, further size reduction of a chip variable resistor is demanded. However, with the structure in which a cylindrical portion is formed at the terminal plate and crimped for fixation to the insulating substrate, there is a limit on the size reduction because of the problems related to the technique of sheet metal working, which provides a limit on the size reduction of a chip variable resistor.
Moreover, in the prior art structure, the rotor is merely pressed and held by the crimped portion of the center cylinder. When the center cylinder or the rotor is worn out at the crimped portion due to the rotation of the rotor, the force to press and hold the rotor by the crimped portion of the center cylinder is considerably reduced. In this case, the rotor may rotate in a subsequent processing step, thereby causing the resistance to deviate from the desired value, or the resistance may fail to be readjustable.
Some of the printed boards on which a chip variable resistor is to be mounted are formed with a through-hole, and there is a demand for the structure which enables the adjustment of the resistance also from the reverse side of the printed board. However, with the prior art structure in which the rotor is attached to the insulating substrate by crimping, the rotational operation of the rotor is possible only from the obverse side of the printed board. Therefore, the structure cannot meet the above demand and lacks adaptability.